What are igneous rocks

N4TdHa Igneous rocks are the rocks that form when molten material cools and crystallizes.

Real-World Reading Link At any given point in time, igneous rocks are forming somewhere on Earth. The location and the conditions that are present determine the types of igneous rocks that form.

Igneous Rock Formation

If you live near an active volcano, you can literally watch igneous rocks form. A hot, molten mass of rock can solidify into solid rock overnight. As you read in Chapter 4, magma is molten rock below Earth's surface. Lava is magma that flows out onto Earth's surface. Igneous rocks form when lava or magma cools and minerals crystallize.

In the laboratory, most rocks must be heated to temperatures of 800°C to 1200°C before they melt. In nature, these temperatures are present in the upper mantle and lower crust. Where does this heat come from? Scientists theorize that the remaining energy from Earth's molten formation and the heat generated from the decay of radioactive elements are the sources of Earth's thermal energy.

Composition of magma The type of igneous rock that forms depends on the composition of the magma. Magma is often a slushy mix of molten rock, dissolved gases, and mineral crystals. The common elements present in magma are the same major elements that are in Earth's crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na). Of all the compounds present in magma, silica is the most abundant and has the greatest effect on magma characteristics. As summarized in Table 5.1, magma is classified as basaltic, andesitic, or rhyolitic, based on the amount of silica it contains. Silica content affects melting temperature and impacts how quickly magma flows.

Interactive Table To explore 1 Types of Magma more about magma composition, visit glencoe.com.

Group

Silica Content

Example Location

Basaltic

42-52%

Hawaiian Islands

Andesitic

52-66%

Cascade Mountains, Andes Mountains

Rhyolitic

more than 66%

Yellowstone National Park

Once magma is free of the overlying pressure of the rock layers around it, dissolved gases are able to escape into the atmosphere. Thus, the chemical composition of lava is slightly different from the chemical composition of the magma from which it developed.

Magma formation Magma can be formed either by melting of Earth's crust or by melting within the mantle. The four main factors involved in the formation of magma are temperature, pressure, water content, and the mineral content of the crust or mantle. Temperature generally increases with depth in Earth's crust. This temperature increase, known as the geothermal gradient, is plotted in Figure 5.1. Oil-well drillers and miners, such as those shown in Figure 5.2, have firsthand experience with the geothermal gradient. Temperatures encountered when drilling deep oil wells can exceed 200°C.

Pressure also increases with depth. This is a result of the weight of overlying rock. Laboratory experiments show that as pressure on a rock increases, its melting point also increases. Thus, a rock that melts at 1100°C at Earth's surface will melt at 1400°C at a depth of 100 km.

The third factor that affects the formation of magma is water content. Rocks and minerals often contain small percentages of water, which changes the melting point of the rocks. As water content increases, the melting point decreases.

^P Reading Check List the main factors involved in magma formation.

Mineral content In order to better understand how the types of elements and compounds present give magma its overall character, it is helpful to discuss this fourth factor in more detail. Different minerals have different melting points. For example, rocks such as basalt, which are formed of olivine, calcium feldspar, and pyroxene (pi RAHK seen), melt at higher temperatures than rocks such as granite, which contain quartz and potassium feldspar. Granite has a melting point that is lower than basalt's melting point because granite contains more water and minerals that melt at lower temperatures. In general, rocks that are rich in iron and magnesium melt at higher temperatures than rocks that contain higher levels of silicon.

Earth's Geothermal Gradient

Earth's Geothermal Gradient

What The Temperature Melt Potassium
Figure 5.1 The average geothermal gradient in the crust is about 25°C/km, but scientists think that it drops sharply in the mantle to as low as 1°C/km.
Earth Depth Temperature
Figure 5.2 The temperature of Earth's upper crust increases with depth by about 30°C for each 1 km. At a depth of 3 km, this drill bit will encounter rock that is close to the temperature of boiling water.

Section 1 • What are igneous rocks? 113

Lowell Georgia/CORBIS

Biotite

Plagioclase

Potassium feldspar

Quartz Hornblende

Solid rock

Solid rock

Molten rock

Potassium feldspar

Biotite

Hornblende

Partially melted rock

Biotite

Plagioclase

Potassium feldspar

Quartz Hornblende

Molten rock

Potassium feldspar

Biotite

Hornblende

Partially melted rock

Figure 5.3 As the temperature increases in an area, minerals begin to melt.

Determine What can you suggest about the melting temperature of quartz based on this diagram?

Partial melting Suppose you froze melted candle wax and water in an ice cube tray. If you took the tray out of the freezer and left it at room temperature, the ice would melt, but the candle wax would not. This is because the two substances have different melting points. Rocks melt in a similar way because the minerals they contain have different melting points. Not all parts of a rock melt at the same temperature. This explains why magma is often a slushy mix of crystals and molten rock. The process whereby some minerals melt at relatively low temperatures while other minerals remain solid is called partial melting. Partial melting is illustrated in Figure 5.3. As each group of minerals melts, different elements are added to the magma mixture thereby changing its composition. If temperatures are not high enough to melt the entire rock, the resulting magma will have a different composition than that of the original rock. This is one way in which different types of igneous rocks form.

^p Reading Check Summarize the formation of magma that has a different chemical composition from the original rock.

Bowen's Reaction Series

In the early 1900s, Canadian geologist N. L. Bowen demonstrated that as magma cools and crystallizes, minerals form in predictable patterns in a process now known as the Bowen's reaction series. Figure 5.4 illustrates the relationship between cooling magma and the formation of minerals that make up igneous rock. Bowen discovered two main patterns, or branches, of crystallization. The right-hand branch is characterized by a continuous, gradual change of mineral compositions in the feldspar group. An abrupt change of mineral type in the iron-magnesium groups characterizes the left-hand branch.

Figure 5.3 As the temperature increases in an area, minerals begin to melt.

Determine What can you suggest about the melting temperature of quartz based on this diagram?

Figure 5.4 On the left side of Bowen's reaction series, minerals rich in iron and magnesium change abruptly as the temperature of the magma decreases. Compare How does this compare to the feldspars on the right side of the diagram?

Magma types *

Basaltic

Andesitic +

Rhyolitic (high silica)

Simultaneous crystallization

Olivine

Olivine

Mica High Heat Feldspar
Potassium feldspar Muscovite mica Quartz

High temperature

Calcium-rich

Low temperature

Iron-rich minerals The left branch of Bowen's reaction series represents the iron-rich minerals. These minerals undergo abrupt changes as magma cools and crystallizes. For example, olivine is the first mineral to crystallize when magma that is rich in iron and magnesium begins to cool. When the temperature decreases enough for a completely new mineral, pyroxene, to form, the oliv-ine that previously formed reacts with the magma and is converted to pyroxene. As the temperature decreases further, similar reactions produce the minerals amphibole and biotite mica.

Feldspars In Bowen's reaction series, the right branch represents the plagioclase feldspars, which undergo a continuous change of composition. As magma cools, the first feldspars to form are rich in calcium. As cooling continues, these feldspars react with magma, and their calcium-rich compositions change to sodium-rich compositions. In some instances, such as when magma cools rapidly, the calcium-rich cores are unable to react completely with the magma. The result is a zoned crystal, as shown in Figure 5.5.

Fractional Crystallization

When magma cools, it crystallizes in the reverse order of partial melting. That is, the first minerals that crystallize from magma are the last minerals that melted during partial melting. This process, called fractional crystallization, is similar to partial melting in that the composition of magma can change. In this case, however, early formed crystals are removed from the magma and cannot react with it. As minerals form and their elements are removed from the remaining magma, it becomes concentrated in silica.

Compare Igneous Rocks

How do igneous rocks differ? Igneous rocks have many different characteristics. Color and crystal size are some of the features that differentiate igneous rocks.

Procedure H] B'i

1. Read and complete the lab safety form.

2. Obtain a set of igneous rock samples from your teacher.

3. Carefully observe the following characteristics of each rock: overall color, crystal size, and, if possible, mineral composition.

4. Design a data table to record your observations. Analysis

1. Classify your samples as either basaltic, andesitic, or rhyolitic. [Hint: The more silica in the rock, the lighter it is in color.]

2. Compare and contrast your samples using the data from the data table. How do they differ? What characteristics do each of the groups share?

3. Speculate in which order the samples crystallized. [Hint: Use Bowen's reaction series as a guide.]

When Magma Cools Images
i Figure 5.5 When magma cools quickly, a feldspar crystal might not have time to react completely with the magma and might retain a calcium-rich core. The result is a crystal with distinct calcium-rich and sodium-rich zones.

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What Picture Igneous Rock

Section 1 • What are igneous rocks? 115

(t)Marli Miller/Visuals Unlimited, (b)Wally Eberhart/Visuals Unlimited

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Responses

  • Saradas
    Which is the most abundant in magma and has the greatest effect on characteristsic?
    8 years ago
  • RALPH
    What the temperature to melt potassium?
    8 years ago
  • adone
    How to make an igneous rock with candle wax?
    8 years ago
  • emppu
    What discovery did N.L Bowen make about the formation of minerals in a cooling magma?
    8 years ago
  • muhammed
    What is the effect of temperature in the formation of igneous rock?
    1 year ago

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